Note: Descriptions are shown in the official language in which they were submitted.
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Description
Method of Forming a Silicon Dioxide Film
Technical Field
The present invention relates to a method of forming
a thick silicon dioxide film on a silicon substrate.
Background Art
A material obtained by forming a thick silicon dioxide
film on a silicon substrate has been used for the devices
that are used in such fields as soft photonics which is chiefly
concerned to optical data communication and processing
technology, amenity photonics which aims at utilizing a light
for the optical display and optical input/output which are
easy for human beings and environment to use, and hard
photonics challenging ultimateness of a new source of light
and developing a technology for utilizing optical
measurement.
An optical waveguide in an optical integrated device,
for example, is formed by burying a core layer having an
optical waveguide circuit pattern in a silicon dioxide film
on a.silicon substrate. In this case, the silicon dioxide
film must have a thickness which is at least not smaller than
ten and several microns if the thickness of the core layer
is selected to be, for example, the almost same size as that
of an optical fiber that is connected.
A typical method of forming the silicon dioxide film
on the silicon substrate can be represented by a well-known
direct thermal oxidation method. The direct thermal
oxidation method is to form a silicon dioxide film by directly
thermally oxidizing the surface of the silicon substrate.
The thickness of the film that is formed varies in proportion
to the time of oxidation when it has a small thickness.
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However, since the oxidation reaction is conducted through
the oxide film that has been formed, formation of the film
having a large thickness takes a time in proportion to the
one-half power of the time of oxidation. It is therefore
difficult to form a film having a thickness of as large as
ten and several microns or more. In order to increase the
rate of oxidation, therefore, there has been put into
practical use a high-pressure oxidation method in an
oxidizing atmosphere of ten and several atms to 25 atms.
However, this method is accompanied by such barriers as legal
restrictions related to high pressures and expensive
facility costs. It is therefore virtually difficult to form
a silicon dioxide layer having a thickness of as large as
ten and several microns to 100 microns or more.
In order to solve the above problems, the present
applicant has developed a method of forming a silicon dioxide
film by the deposition of polysilicon (Japanese Patent
Application No. 342893/2000 "Method of Forming a Silicon
Dioxide Film and Method of Forming Optical Waveguide"). This
method is to form a silicon dioxide layer of a desired
thickness by depositing polysilicon on a silicon substrate,
followed by thermal oxidation treatment to form a silicon
dioxide film, then, newly depositing polysilicon on the thus
formed silicon dioxide film, followed by thermal oxidation
treatment to form a silicon dioxide film, and repeating the
above operations.
Disclosure of the Invention:
Formation of the silicon dioxide film by the deposition
of polysilicon, however, involves problems that must be
improved as described below.
(1) Surface roughness of the silicon dioxide film:
The silicon dioxide film formed by the deposition of
polysilicon has relatively rough surfaces. When, for
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example, a silicon dioxide film is deposited on these
surfaces for forming an optical waveguide core layer, a rough
surface exists on the boundary surface causing light to be
scattered and inviting a possibility of increasing optical
loss. As required, therefore, the surfaces of the silicon
dioxide film are flattened prior to forming the core layer
to improve the surface roughness.
(2) Productivity:
A thick film formed by the deposition of polysilicon
is favorable for forming a silicon dioxide film on a large
quantity of wafers. However, formation of the film having
a thickness of, for example, ten and several microns requires
a time of several days though this is still quicker than the
method based on the direct thermal oxidation. When there is
employed a reduced-pressure vapor-phase deposition method
which is a typical silicon film deposition method, it becomes
necessary to use a production facility working under a
pressure reduced to a large degree. It has, therefore, been
desired to provide a production system capable of suitably
meeting a variety of customer's requirements for forming the
silicon dioxide film, a requested delivery time and a
quantity of production, and others.
The present invention has been accomplished in view
of the above-mentioned fact, and its technical subject is
to provide a method of forming a silicon dioxide film, which
comprises repeating steps of depositing a silicon layer on
a silicon substrate and then, subjecting the silicon layer
to a thermal oxidation treatment to form a silicon dioxide
film of a predetermined thickness, and which makes it
possible to suitably select the surface roughness of the
silicon dioxide film that is formed and the rate of growth
of the silicon film that is deposited.
As a method of forming a silicon dioxide film to solve
the above technical subject, according to the present
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invention, there is provided a method of forming a silicon
dioxide film, comprising the steps of:
a deposition step of depositing any one of polysilicon,
epitaxial silicon or amorphous silicon on a silicon substrate
or on the silicon dioxide film formed on the silicon substrate
by a thermal oxidation treatment; and
a thermal oxidation step of oxidizing the silicon film
by heat to convert it into a silicon dioxide film;
the step of deposition and the step of thermal oxidation
being repeated a plural number of times.
The silicon film is deposited by selecting any one of
polysilicon deposition, epitaxial silicon deposition or
amorphous silicon deposition, or by selecting a combination
thereof so that the surface roughness and the rate of growth
of the silicon dioxide film that is formed can be suitably
changed.
Desirably, the thickness of the silicon film formed
per one time of the deposition step is not larger than 5,u.
M.
Brief Description of the Drawin_gs:
Fig. 1 is a diagram illustrating the steps of a method
of forming a silicon dioxide film according to the present
invention; and
Fig. 2 is a table showing one example of results of
the experiment.
Best Mode for Carrying Out the Invention:
A method of forming a silicon dioxide film according
to the present invention will now be described in detail with
reference to the case of forming a silicon dioxide film having
a thickness of 15 um as one example.
(1) Thermal oxidation of silicon substrate:
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A silicon substrate 2 shown in Fig. 1(a) is oxidized
by heat in an electric furnace to form a silicon dioxide film
4 on a surface of the silicon substrate 2 as shown in Fig.
1(b). The thermal oxidation is conducted according to a wet
5 oxidation method using, for example, water vapor. The wet
oxidation can be carried out at a temperature of 1080 C that
is fully lower than a melting point which is 1410 C of the
silicon substrate 2, without requiring the treatment under
a high pressure. In this step of thermal oxidation, there
is formed the silicon dioxide film having a thickness of,
for example, 2 ,(,Cm.
(2) Step of deposition:
As shown in Fig. 1(c), on the above silicon dioxide
film 4 is deposited polysilicon, epitaxial silicon or
amorphous silicon by a known chemical vapor-phase deposition
method (CVD) to form a silicon film 6. As the chemical
vapor-phase deposition method, there is used a typical
reduced-pressure vapor-phase deposition method. The
selection of polysilicon, epitaxial silicon or amorphous
silicon and the reduced-pressure vapor-phase deposition
method will be described later. In this step, there is formed
a silicon-deposited film having a thickness of, for example,
1gm. It is desired that the thickness of the film is not
larger than 5,um so that a large rate of oxidation can be
obtained in the next step of thermal oxidation.
(3) Step of thermal oxidation:
Like the thermal oxidation of the silicon substrate
in (1) above, the silicon film 6 is oxidized by heat in the
electric furnace to form, as shown in Fig. 1(d), a silicon
dioxide film 8 of the same quality on the first silicon dioxide
film 4. The above silicon film 6 of the thickness of 1
m turns into the silicon dioxide film 8 of a thickness of
3,um due to the expansion of volume by the thermal oxidation.
(4) Repetition of the step of deposition and the step of
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thermal oxidation:
As shown in Figs. 1(e) and 1(f), the above step of
deposition and the step of thermal oxidation are repeated
until there is formed a silicon dioxide film having a
predetermined thickness T of 15 ,c.[m. The film thickness T
of 15 um is obtained by repeating the step of deposition and
the step of thermal oxidation five times, respectively.
Namely, thermal oxidation of the substrate 2,c.cm + (3,um x
4 t ime s) + (1 gm x 1 t ime )= 15 ,(.L m.
The final film of the thickness of 1,um is formed by
thermally oxidizing the silicon film having a thickness of
0.3 ,c..c m .
(5) Deposition of silicon by the reduced-pressure vapor-
phase deposition method:
The deposition of polysilicon, epitaxial silicon and
amorphous silicon by the reduced-pressure vapor-phase
deposition method will now be described with reference to
Fig. 2 which shows one example of results of experiment
conducted by the present inventor.
(5-1) Deposition of polysilicon:
Deposition of polysilicon makes it possible to form
a polycrystalline silicon film in a manner as described
below.
Under the conditions of a monosilane gas ( SiH4 ) of 100%,
80 cc/min, 570 C and 100 Pa (0.75 Torr), the film grew at a
rate of 0.0020 ,c,Cm/min and the surface roughness RMS after
the thermal oxidation treatment was 20.5 nm.
(5-2) Deposition of epitaxial silicon:
Deposition of epitaxial silicon makes it possible to
form a film of monocrystalline silicon by the silicon
epitaxial growth in a manner as described below.
Under the conditions of a monosilane gas ( SiH4 ) of 20%,
1200 cc/min, 1000 C and 5320 Pa (40 Torr), the film grew at
a rate of 1.06 /im/min and the surface roughness RMS after
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the thermal oxidation treatment was 0.15 nm.
(5-3) Deposition of amorphous silicon:
Deposition of amorphous silicon makes it possible to
form an amorphous film in a manner as described below.
Under the conditions of a monosilane gas (SiH4) of 100% ,
80 cc/min, 520 C and 100 Pa (0.75 Torr), the film grew at a
rate of 0.0013 ,um/min and the surface roughness RMS after
the thermal oxidation treatment was 0.4 to 0.7 nm. It is
possible to form a film over a relatively large area.
Accordingly, when it becomes necessary to improve the
surface roughness of the silicon dioxide film on the way to
form the silicon dioxide film of a predetermined thickness
T (15 ,c.cm), the silicon film may be deposited by selecting
either epitaxial silicon or amorphous silicon. To quicken
the rate of growing the silicon film, epitaxial silicon may
be selected. Epitaxial silicon deposition can be performed
at a reduced pressure to a relatively small degree and an
apparatus therefor is relatively simple. When amorphous
silicon is deposited, there can be formed a film over a
relatively large area.
Described below are functions of the above-mentioned
methods of forming the silicon dioxide film.
(1) Surface roughness and rate of growth of the silicon
dioxide film:
The deposition of epitaxial silicon or the deposition
of amorphous silicon makes it possible to improve the surface
roughness of the silicon dioxide film, which has been a
problem in the deposition of polysilicon. Namely, the
surface roughness (RiMS) is greatly improved to 0.15 nm in the
case of epitaxial silicon and 0.4 to 0.7 nm in the case of
amorphous silicon as compared with 20.5 nm in the case of
polysilicon. Further, the rate of growing the silicon
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film is greatly improved to 1.06 um/min when epitaxial
silicon is deposited as compared with 0.0020 um/min of when
polysilicon is deposited. By epitaxial silicon deposition,
therefore, the time for growing the film can be shortened.
(2) Productivity:
Any one of polysilicon, epitaxial silicon or amorphous
silicon is appropriately selected to be deposited to form
a thick silicon dioxide film on the silicon substrate
depending upon such requirements as production in large
amounts, production in small amounts, facility that can be
utilized, requested delivery time, requested surface quality,
and sizes of the surfaces to be formed.
(3) Combination of silicon depositions:
Further, on the way to form a thick silicon dioxide
film, there may be suitably deposited polysilicon, epitaxial
silicon or amorphous silicon in a suitable combination as
required.
According to the method of forming a silicon dioxide
film constituted by the present invention, a silicon dioxide
film having a predetermined thickness can be form by
repeating the steps of depositing a silicon layer on a silicon
substrate, and then, subjecting the deposited silicon layer
to the thermal oxidation treatment to form a silicon dioxide
film, and further, the surface roughness of the silicon
dioxide film that is formed and the rate of growing the silicon
film that is deposited can be appropriately selected.
